wangyu- / test

1. test

asdasdasd

和差公式 (由欧拉公式可推)
$\sin (x + y) = \sin (x)\cos (y) + \cos (x)\sin (y)$
$\cos (x + y) = \cos (x)\cos (y) - \sin (x)\sin (y)$

积化和差（由和差公式直接得出）
<math>\sin\theta\sin\phi={\cos(\theta-\phi)-\cos(\theta+\phi)\over2}</math>
<math>\cos\theta\cos\phi={\cos(\theta-\phi)+\cos(\theta+\phi)\over2}</math>
<math>\sin\theta\cos\phi={\sin(\theta+\phi)+\sin(\theta-\phi)\over2}</math>
<math>\cos\theta\sin\phi={\sin(\theta+\phi)-\sin(\theta-\phi)\over2}</math>